In MR imaging, pulse sequences consisting of RF pulses and switched magnetic field gradients are applied to an object (a patient) to generate magnetic resonance signals, which are scanned in order to obtain information therefrom and to reconstruct images of the object. Since its initial development, the number of clinical relevant fields of application of MRI has grown enormously. MRI can be applied to almost every part of the body, and it can be used to obtain information about a number of important functions of the human body. The pulse sequence which is applied during an MRI scan determines completely the characteristics of the reconstructed images, such as location and orientation of the image slice in the object, dimensions, resolution, signal-to-noise ratio, contrast, sensitivity for movements, etcetera. An operator of a MRI device has to choose the appropriate sequence and has to adjust and optimize its parameters for the respective application.
There are several applications in MR imaging that require the acquisition of a whole slice within the examined body in one single shot. In that case, usually EPI (Echo Planar Imaging) is the method of choice. EPI typically uses an initial spatially selective 90° RF pulse to excite the nuclear magnetization within the image slice of interest. The initial pulse generates an echo signal which is thereafter repetitively refocused by read-out magnetic field gradients of quickly alternating polarity to form a train of multiple gradient echoes. Each of these gradient echoes is typically differently phase encoded by additional small gradient pulses occurring between the echoes. While an EPI sequence can collect a full MR image data set in a very short time (e.g., tens of milliseconds), it requires comparatively high performance hardware of the used MR device. For high field MR imaging, EPI is particularly interesting because it is very efficient in terms of received signal power over transmitted RF power (SAR). Unfortunately, EPI is susceptible to significant image distortion due to main magnetic field inhomogeneity, T2 relaxation, and chemical shift effects that evolve over the relatively long duration of the echo train. This is a particularly serious drawback of EPI in cases in which it is desired to achieve high image resolution. The reason is that an increase of image resolution always implies a corresponding increase of the duration of the EPI echo train.
It is well known that the afore-mentioned problems and drawbacks of the EPI sequence can be resolved by the incorporation of additional RF pulses into the imaging sequence. Such methods are known as multi-shot EPI or GRASE, as it is described, e.g., in the document U.S. Pat. No. 5,270,654. The main disadvantage of these known techniques is their much higher RF energy deposition (SAR), which can easily exceed presently acceptable safety limits for the human body.